We have a variety of English translations for several reasons. The first is that whenever a document is translated from one language to another, it is impossible to do a word-for-word translation. Different languages seldom have identical word meanings or grammatical structures. Therefore, different translations usually represent different styles of translation. Using some popular English translations as examples: the King James Version uses elegant but often old-fashioned English; the New American Standard Bible strives to be as close as possible to a word-for-word translation while still retaining normal English syntax; the Living Bible uses paraphrasing to communicate the meaning of the text; and the NIV utilizes a thought-for-thought or idea-for-idea method of translation called dynamic equivalence.
A second reason for new translations is that languages are constantly changing. Meanings of individual words and ways of expressing concepts are always in flux. This is why the original King James Version (written in the 1600s) is difficult for many modern readers to understand. In fact, the English language changed so much over the next 150 years, that the King James Version we read today underwent numerous modifications until 1769.
Finally, there are large numbers of ancient manuscripts in the original languages, and they contain some minor differences. Nearly all conservative scholars agree that these differences affect word choices, but not major doctrines.